1. Field of the Invention
This invention relates to an earth boring tool commonly known as an underreamer and, more particularly, to such a tool having an alternate fluid flow path therethrough for flushing debris from beneath the extended arms of the tool to permit retraction of the arms to within the body of the tool.
2. Brief Description of the Prior Art
Underreamers are well known tools used in the well drilling art to enlarge portions of a borehole in oil and gas wells for various purposes. The underreamer generally comprises a tubular body having a pilot cutter or underreaming lugs at one end and threaded means at the opposite end for connection to a drill string. The tubular body includes a plurality of axially elongated cavities. An elongated cutter arm is housed within each cavity with the lower end of the arm supporting a rotating cone cutter and the opposite end hinged to the body through a hinge pin assembly for rotational movement of the arm from within the cavity to an extended position wherein the cutter is effective to contact the wall of the borehole and enlarge it.
The tool also includes an axially moveable central wash pipe in fluid flow communication with the drilling mud for circulating mud to the borehole to flush the cuttings therefrom. The washpipe normally includes a piston having a limited size opening therethrough, in sliding sealing engagement with the central axial bore of the tubular body, and a pipe portion extending therefrom with a jet nozzle at its lower end for jetting the drilling fluid therefrom. A cam member defining a plurality of cam lobes is attached to the pipe adjacent the lower end thereof with each lobe in alignmet with cam follower surfaces defined on the back faces of the hinged arms so that as the wash pipe moves axially downwardly under the influence of an increase in pressure of the drilling fluid on the face of the piston, the cam lobes contact the surfaces and force the arms from a retracted position generally flush with the surface of the tubular body, to the full extended position.
A spring member is disposed below the piston and a lower shoulder in the internal bore of the body and normally biases the piston to its axially upward position corresponding to the cam lobes permitting the arms to be in the retracted position. Thus, when the underreamer operation is completed, the mud pressure is reduced to normal to permit the spring to return the piston to this position. In such position the arms are permitted to return to the retracted position under their own weight.
However, it can be appreciated, that with the arms enlarging the borehole, the circulating drilling mud in the vicinity of the arms does not necessarily flow past the extended arms and cutters with sufficient velocity and controlled flow pattern to insure that the cuttings will be removed from between the extended arms and the body or from the arm cavities and hinge area. As a matter of fact, it is the general experience that, without some special fluid flow path for flushing this specific area, it is not uncommon for the cuttings to prevent the full retraction of the arms. Further, during withdrawal of the tool from the borehole, it is not always apparent to the drilling personnel that the arms are not retracted to the flush position and thus, the extended arms, upon withdrawal engage the borehole as it is being withdrawn through the portion that was not underreamed.
It is thus readily apparent that upon retracting the tool from the borehole, as the downwardly outwardly extending arms contact the narrow portion of the borehole or casing, the upward withdrawing force places considerable force on the arms to retract them to the flush position. However, if cuttings or debris such as compacted gumbo or rock fragments interfere with the closing, this force is transmitted to the hinge assembly and multiplied by the leverage of the arm about the obstruction. This in turn has resulted in the damage and total destruction of the hinge assembly, and can even cause loss of the arm downhole.
It is known to continuously divert a portion of the drilling fluid to and through the hinge assembly as shown in U.S. Pat. No. 1,899,727 or jetted, through directional jets from the wash pipe toward the underarm area. In that such jets or flow paths are not consistent with providing the maximum flow for flushig the borehole, although they provide some useful debris dislodging, they are not sufficiently dependable.